| Year | Value | | 2021 | 3.2410131145701 | | 2020 | 3.2586222318242 | | 2019 | 3.3094941042006 | | 2018 | 3.3366884575441 | | 2017 | 3.3746034018739 | | 2016 | 3.4261455847484 | | 2015 | 3.4609934389969 | | 2014 | 3.5076312699814 | | 2013 | 3.5248178372355 | | 2012 | 3.5292910968348 | | 2011 | 3.5558495646572 | | 2010 | 3.5791838866183 | | 2009 | 3.5988849378891 | | 2008 | 3.6200318412964 | | 2007 | 3.6321513296143 | | 2006 | 3.649967184262 | | 2005 | 3.6294288876853 | | 2004 | 3.6817131566704 | | 2003 | 3.7171767646244 | | 2002 | 3.7547329983007 | | 2001 | 3.799084478979 | | 2000 | 3.8267446834648 | | 1999 | 3.9029758557187 | | 1998 | 3.9684674722251 | | 1997 | 4.0353173757651 | | 1996 | 4.1094981321167 | | 1995 | 4.1299155745329 | | 1994 | 4.1764599219406 | | 1993 | 4.2166951443871 | | 1992 | 4.2561395196662 | | 1991 | 4.291506856262 | | 1990 | 4.3230586322831 | | 1989 | 4.3773569316985 | | 1988 | 4.4326568491009 | | 1987 | 4.4958572712063 | | 1986 | 4.5601392524532 | | 1985 | 4.6428895682147 | | 1984 | 4.7305138450099 | | 1983 | 4.8138326901602 | | 1982 | 4.8926099893581 | | 1981 | 4.9592118614906 | | 1980 | 5.0256097800603 | | 1979 | 5.0897765704926 | | 1978 | 5.1342213785094 | | 1977 | 5.1805247307031 | | 1976 | 5.2277754867994 | | 1975 | 5.2676664472313 | | 1974 | 5.2972579438483 | | 1973 | 5.3765866736619 | | 1972 | 5.4831415967381 | | 1971 | 5.6179926057918 | | 1970 | 5.7426489782205 | | 1969 | 5.8634702406956 | | 1968 | 5.983494429032 | | 1967 | 6.1169365814965 | | 1966 | 6.2291112617436 | | 1965 | 6.3345711230005 | | 1964 | 6.4385458204746 | | 1963 | 6.5123753038568 | | 1962 | 6.5823795020893 | | 1961 | 6.6832155100931 | | 1960 | 6.7682360569083 |
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